The subject matter disclosed herein relates generally to magnetic resonance imaging (MRI) systems, and more particularly to a distributed capacitance radio frequency (RF) coil for MRI systems.
MRI is a medical imaging modality that generates images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”) and gradient coils to produce smaller amplitude, spatially varying magnetic fields when a current is applied to the gradient coils. When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with hydrogen nuclei in tissue water become polarized. The magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention) and the gradient coils encode the MR signal.
RF coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei, also referred to herein as the Larmor frequency. These RF coils are used to transmit RF excitation signals and receive MR signals used to form the images. Various types of RF coils may be used in an MRI system such as a whole-body RF coils and RF surface (or local) coils. Two common RF coil configurations are the birdcage coil and the transverse electromagnetic (TEM) coil. The birdcage coil typically includes a pair of end rings and a plurality of rungs coupled between the pair of end rings. In order to reduce a volume of the birdcage coil, a portion of the end rings and/or rungs, of at least one known birdcage coil, are fabricated from a thin copper foil.
Under various operational conditions, the known birdcage coil may be subjected to relatively high power, for example, 7 Tesla (T). When operating at the relatively high power, air between gaps of the rungs and/or end rings may ionize. More specifically, when the voltage on the known RF coil is increased to a sufficient level the air is converted from an insulator to a conductor, also referred to herein as the breakdown voltage. Thus, when the RF coil reaches the breakdown voltage, ionization of the air in the gaps may result in arcing to occur between various portions of the birdcage coil which may result in an eventual failure of the RF coil.